The invention relates to improvements in electrical machines which employ carbon brushes, and more particularly to improvements in apparatus for monitoring the condition of brushes in such machines.
It is known to install two or more carbon brushes on a common brush holder which is mounted in an electrical machine in such position that the brushes can engage with the commutator or slip ring of the machine. The brushes undergo wear and must be replaced at certain intervals when the amount of wear reaches a maximum permissible value. As a rule, the brushes are biased against the commutator or slip ring by a spring-biased follower means so as to ensure that each brush will remain in contact with the adjacent part of the machine regardless of the amount of wear upon the brush.
Timely replacement of spent brushes is desirable and advantageous because this reduces the likelihood of an abrupt interruption of operation at an inopportune time, e.g., when the machine is in actual use, and/or damage to the machine. Spent brushes could damage the collector of an electrical machine.
In accordance with a presently known proposal, the machine is equipped with a stop which prevents further forward movement of a brush when the wear upon such brush reaches the maximum permissible value, i.e., when the length of the brush is reduced to a minimum permissible value.
In accordance with a different prior proposal, the follower which biases the brush against a commutator or against a slip ring carries an insulated spring which engages a complementary contact on a rail of the brush holder to thereby initiate the generation of a signal denoting that the wear upon the brush has reached the maximum permissible value. A drawback of such monitoring devices is that their contacts are exposed to dust, which develops as a result of wear upon the brushes, and to other foreign matter in the surrounding atmosphere. This can result in premature or delayed generation of signals with attendant potential damage to the commutator or to another part of the machine and/or in abrupt interruption of operation of the machine. Another drawback of such monitoring devices is that the contacts are not readily accessible and can be manipulated only by specially designed tools. Still further, the exact positioning of contacts is different for each type of brush holders. Moreover, the rails and the contacts take up a substantial amount of space which is not available on relatively small brush holders so that the utilization of such monitoring devices is limited to certain large types of brush holders and electrical machines. Reference may be had to German Utility Model No. 82 34 555.4.
German Offenlegungsschrift No. 34 44 482 discloses a modified monitoring device which employs a microswitch. This contributes to greater reliability of the signal generation. However, the monitoring device which is disclosed in this printed publication cannot be used with all types of brush holders because it takes up too much space so that it cannot monitor the extent of wear of brushes in a relatively small brush holder, especially if the brush holder is designed to carry several closely adjacent brushes.